High-efficiency miniature magnetic integrated circuit structures

ABSTRACT

The present invention, generally speaking, provides a magnetic memory element that is single domain in nature and has a geometry that mitigates the effects of half-select noise. In a preferred embodiment, the magnetic memory element takes the form of a magnetic post or tube having an aspect ratio in the range of 2:1 (more preferably 4:1). The outside diameter of the magnetic tube or post is preferably less than 0.8 microns, more preferably 0.6 microns or less. The magnetic post or tube then functions as a single magnetic domain. In the case of a magnetic tube, the skin of the tube is formed of a magnetic material and the interior of the tube is formed of a non-magnetic material. Suitable non-magnetic materials include copper, gold and silicon. The coercivity of the magnetic tube structure may be adjusted by adjusting the thickness of the magnetic skin. As a result, the magnetic memory element is readily scalable to smaller geometries as lithographic techniques improve. The combination of very small, single-domain size and a relatively large aspect ratio results in uniquely desirable properties. Current levels within any reasonable expectation operate to switch the state of the magnetic tube only when the magnetic tube is destabilized by running current through it. With current flowing through the magnetic tube, its state may be readily changed by running modest currents in opposite directions through two parallel conductors, one on each side of the magnetic tube. When the magnetic tube is switched, the single domain nature of the magnetic tube produces a signal that is typically 10-15 times stronger than signals produced by conventional magnetic memory elements. The magnetic tube functions as a vertical magnetic field generator and may be formed in intimate proximity to a magnetic field sensor such as above the gate of a magFET.

FIELD OF THE INVENTION

The present invention relates to magnetic integrated circuit structuressuitable for use in magnetic semiconductor memories.

STATE OF THE ART

Computer memory technology has experienced profound advances in thecourse of the last two decades. An early enabling computer memorytechnology was magnetic core memory technology. To form magnetic corememories, seemingly innumerable miniature ferrite cores, toroid-shaped,were painstakingly interwoven within a fine matrix of wires, three ormore wires passing through the center of each core. By applying amagnetizing current, each core could be placed in one of two differentmagnetic states, representing a logic 1 and a logic 0, respectively. Acoincident current technique was used to select particular cores for thereading and writing of data. Core memory is non-volatile, meaning thatdata remains unchanged over power cycles: power can be removed from thememory and later reapplied without changing the contents of the memory.Core memory is also "radiation-hard," i.e., unaffected in its operationby ionizing radiation such as gamma rays. Nevertheless, by and large,because of the labor-intensive nature of core memory manufacture and itssize, core memory has long been abandoned in favor of semiconductormemory.

Currently the most popular memory technology is MOS DRAM(Metal-Oxide-Semiconductor Dynamic Random Access Memory) technology. InMOS DRAMs, a data bit is stored by injecting charge into or removingcharge from a single storage capacitor, through a single transistor.Very dense MOS DRAM chips of up to 256 Mb capacity have been achieved.DRAMs, like MOS devices generally, are low in power consumption. Becauseleakage occurs from the storage capacitor, however, DRAMs must berefreshed (have the correct charge on each individual storage capacitorrestored) at frequent intervals. Furthermore, as compared to core memorywhich is non-volatile and radiation-hard, DRAM is neither. The need fornon-volatile memory and rapid-access memory therefore requires the useof memory hierarchies including multiple different types of memory,i.e., both non-volatile memory (such as FLASH, EEPROM, Ferroelectricmemory, EPROM disk, tape, etc.) and volatile memory (such as DRAM, SRAM,etc.). Flash, EEPROM and Ferroelectric memory types providenon-volatility but have a limited number of write cycles before wear-outand in general the write cycle is substantially slower than the readcycle. Memory hierarchies in turn require memory I/O architectures ofvarying degrees of sophistication. In the case of personal computers,starting up, or "booting" the computer can take a considerable amount oftime as a consequence of the need to transfer information fromnon-volatile to volatile memory.

Clearly, a non-volatile semiconductor memory comparable to DRAM in termsof density, power consumption and write cycle speed is much to bedesired. Several memory structures have been proposed to this end, asexemplified by U.S. Pat. Nos. 5,329,480, 5,295,097, 5,068,826,4,887,236, 4,803,658, and 3,727,199, among others. Unfortunately,efforts to perfect such magnetic semiconductor memories have beenlargely unsuccessful. One impediment has been the low sensitivity ofprior-art magnetic field sensing devices and the inability to make smallintegratable permanent magnets. A well-known sensing mechanism is thatof carrier deflection, using a device known as a "magFET," for example.A magFET is a Field Effect Transistor (FET) having a single source andtwo or more drains. A magnetic field, when present in the channel regionof the device, can deflect carriers away from one of the drains andtoward the other drain(s), depending on the strength and direction ofthe magnetic field.

More particularly, charge carriers passing through a magnetic fieldexperience a force known as the Lorentz force that tends to deflect thecarriers according to the direction of the magnetic field. The Lorentzforce (F) experienced by a charged carrier is given by the vectorequation F=qv×B, where q is the charge of the carrier, v is the velocityof the carrier and B is the magnetic field through which the carrier ispassing (v & B being tensor quantities). (See, for example, C. S.Roumenin, Handbook of Sensors and Actuators, Volume 2, Section 1.3). TheLorentz force, and hence carrier deflection, is maximized when themagnetic field is substantially orthogonal to the direction of carriertravel. In general, the prior art falls far short of achieving anorthogonal field of sufficient magnitude to allow an integrated sensorto generate a detectable signal.

Other shortcomings of proposed prior-art magnetic semiconductor memoriesalso appear. In order to compete successfully with DRAM, a magneticsemiconductor memory bit cell (after giving effect to the gain providedby additional turns in the magnetic structure) should require no moremagnetizing current through its permanent magnet structure of severalturns than is available from a single, small-size MOS device, which isusually on the order of 2 milliamps or less. Furthermore, the magneticsof such a cell must be sufficiently well-behaved as to not adverselyaffect adjacent cells in a memory array. In general, the prior art hasbeen unable to provide a magnetic semiconductor element that satisfiesall commercial requisites.

SUMMARY OF THE INVENTION

The present invention, generally speaking, provides a magnetic memoryelement that is single domain in nature and has a geometry thatmitigates the effects of half-select noise. In a preferred embodiment,the magnetic memory element takes the form of a magnetic post or tubehaving an aspect ratio in the range of 2:1 (more preferably 4:1). Theoutside diameter of the magnetic tube or post is preferably less than0.8 microns, more preferably 0.6 microns or less. The magnetic post ortube then functions as a single magnetic domain. In the case of amagnetic tube, the skin of the tube is formed of a magnetic material andthe interior of the tube is formed of a non-magnetic material. Suitablenon-magnetic materials include copper, gold and silicon. The coercivityof the magnetic tube structure may be adjusted by adjusting thethickness of the magnetic skin. As a result, the magnetic memory elementis readily scalable to smaller geometries as lithographic techniquesimprove. The combination of very small, single-domain size and arelatively large aspect ratio results in uniquely desirable properties.Current levels within any reasonable expectation operate to switch thestate of the magnetic tube only when the magnetic tube is destabilizedby running current through it. With current flowing through the magnetictube, its state may be readily changed by running modest currents inopposite directions through two parallel conductors, one on each side ofthe magnetic tube. When the magnetic tube is switched, the single domainnature of the magnetic tube produces a signal that is typically 5-15times stronger than signals produced by conventional magnetic memoryelements. The magnetic tube functions as a vertical magnetic fieldgenerator and may be formed in intimate proximity to a magnetic fieldsensor such as above the gate of a magFET.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be further understood from the followingdescription in conjunction with the appended drawing. In the drawing:

FIG. 1 is a perspective view of one magnetic integrated circuitstructure suitable for use in magnetic semiconductor memories;

FIG. 2 is a plan view of the magnetic integrated circuit structure ofFIG. 1;

FIG. 3 is a sectional view along section III--III of the magneticintegrated circuit structure of FIG. 2;

FIG. 4 is a sectional view along section IV--IV of the magneticintegrated circuit structure of FIG. 2;

FIG. 5 is a perspective view of another magnetic integrated circuitstructure suitable for use in magnetic semiconductor memories;

FIG. 6 differs from FIG. 5 in that the core of the solenoid is shown indashed outlines;

FIG. 7 differs from FIG. 6 in that only the coil of the solenoid isshown;

FIG. 8 is an exploded view of a further magnetic integrated circuitstructure suitable for use in magnetic semiconductor memories;

FIG. 9 is an isometric view of the magnetic integrated circuit structureof FIG. 8;

FIG. 10 is a rotated exploded view of the magnetic integrated circuitstructure of FIG. 8;

FIG. 11 is a rotated isometric view of the magnetic integrated circuitstructure of FIG. 8;

FIG. 12 is a plan view of the magnetic integrated circuit structure ofFIG. 8;

FIG. 13A, FIG. 13B and FIG. 13C are schematic diagrams of one magneticsemiconductor memory cell in which the magnetic integrated circuitstructure of FIG. 5 or FIG. 8 is or can be combined with oneconventional magnetic field sensor arrangement;

FIG. 14 is a schematic diagram of one magnetic semiconductor memory cellin which the magnetic integrated circuit structure of FIG. 5 or FIG. 8is or can be combined with another conventional magnetic field sensorarrangement;

FIG. 15 is a perspective view of a magnetic semiconductor memory cellarray using the magnetic integrated circuit structure of FIG. 1;

FIG. 16 is a plan view of the magnetic semiconductor memory cell arrayof FIG. 15;

FIG. 17 is a sectional view along section XVII--XVII of the magneticsemiconductor memory cell array of FIG. 16;

FIG. 18 is a sectional view along section XVIII--XVIII of the magneticsemiconductor memory cell array of FIG. 16;

FIG. 19 is a perspective view of a magnetic semiconductor memory cellarray using the magnetic integrated circuit structure of FIG. 5;

FIG. 20 is an equivalent circuit diagram of a cell of the magneticsemiconductor memory cell array of FIG. 19;

FIG. 21 is a list of process flow steps for forming a magneticintegrated circuit structure like that of FIG. 1, in accordance with onemethod;

FIG. 22 is a cross-sectional view, corresponding to the process flow ofFIG. 21 of a magnetic integrated circuit structure like that of FIG. 1;

FIG. 23 is a list of process flow steps for forming a magneticintegrated circuit structure like that of FIG. 5, in accordance with onemethod;

FIG. 24 is a cross-sectional view corresponding to the process flow ofFIG. 23, of a magnetic integrated circuit structure like that of FIG. 5;

FIG. 25 is a plan view corresponding to the cross-sectional view of FIG.24;

FIG. 26 is a schematic diagram of a buffer circuit coupled to a memorycell of the present invention;

FIG. 27 is a perspective view of another magnetic integrated circuitstructure suitable for use in magnetic semiconductor memories;

FIG. 28 is a perspective view of a magnetic semiconductor memory cellarray using the magnetic integrated circuit structure of FIG. 27;

FIG. 29 is a list of process flow steps for forming a magneticintegrated circuit structure like that of FIG. 27, in accordance withone method; and

FIG. 30 is an equivalent circuit diagram of a cell of magneticsemiconductor memory cell array of FIG. 28, with associated circuitry.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a perspective view of onemagnetic integrated circuit structure suitable for use in magneticsemiconductor memories. The magnetic integrated circuit structure 100 isformed on a semiconductor (e.g., silicon) substrate 101 and incorporatesa magFET, i.e., an FET having a single source region contacted by asource contact 102, a gate electrode 103, and multiple drains (two areshown) contacted by respective drain contacts 105, 106. The sourcecontact is joined to a source line 112. The respective drain contacts105, 106 are joined to respective drain lines 104, 108. The magneticintegrated circuit 100 can differ from a conventional magFET, however,in several important respects. Unlike a conventional magFET, the gateelectrode 103 of the magnetic integrated circuit 100 is formed of aferromagnetic material. The gate electrode, or "ferromagnetic gate" 103,is separated from the underlying device channel by a gate oxide layer107 having a thickness of only a few tens of angstroms. Because of theferromagnetic gate's very close physical proximity to the devicechannel, when the ferromagnetic gate is magnetically coupled to afurther cooperating ferromagnetic member, a magnetic field may bedirected through the device channel with only a small loss of magneticflux. Among other respects, the magnetic integrated circuit structure100 differs from the prior art such as U.S. Pat. No. 4,803,658 in thatthe cooperating ferromagnetic member functions as a vertical magneticfield generator. The '658 patent relies on the Hall effect which is aresult of the Lorentz force, whereas in the present structure theLorentz force itself is used for deflection.

Various cooperating ferromagnetic members are described herein that canfunction with a conventional two- or three-drain magFET or with twodimensional electron gas interfaces of heterojunctions. A twodimensional electron gas is the thinnest sheet of charge currently known(even thinner than the typical 100 Å inversion layer). It is thoughtthat it will be highly sensitive to magnetic fields. The present magnetstructures may be expected to strongly deflect particles in theso-called electron gas formed at the interface of two dissimilarmaterials (i.e., a heterojunction). The magFET may or may not have aferromagnetic gate. As shown in FIG. 1, one such cooperatingferromagnetic member 109 underlies the device channel so as to opposethe ferromagnetic gate. The cooperating ferromagnetic member 109 isjoined to the ferromagnetic gate 103 to form a C-shaped member 110. TheC-shaped member 110 surrounds and "clamps" the carrier channel of themagFET incorporated within the structure 100. A gap 111 between theopposite edges of the C-shaped member forms a very narrow magnetic fluxgap. The device source is centered within the length of the flux gap.Because the ratio of the length of the C-shaped flux path to the gap canbe large and because the fringe field is small, the magnetic efficiencyof the structure 100 is high.

A plan view of the structure 100 of FIG. 1 is shown in FIG. 2. Indicatedin the plan view of FIG. 2 are sectional views III--III and IV--IV,shown in FIG. 3 and FIG. 4, respectively.

Referring more particularly to FIG. 3, the C-clamp member 110, formed ofthe ferromagnetic gate 103 and the cooperating ferromagnetic member 109is clearly seen. The cooperating ferromagnetic member 109 is formedwithin the semiconductor substrate 101 using techniques described ingreater detail hereinafter. A source diffusion region 312 is formedwithin the semiconductor substrate 101. The ferromagnetic gate 103 isformed above the thin gate oxide layer 107, which is part of the thickeroxide layer 315. The contact 102 to the source diffusion 312 is formedwithin the oxide layer 315. Of the two drain lines 104 and 108, thedrain line 108 may be seen in FIG. 3, formed above the oxide layer 315.Although the lateral locations of the drains have been shown forreference, neither drain is actually visible in FIG. 3.

In FIG. 4, one of the two drain contacts is visible. The drain contact105 extends down from the drain line and through a "notch" in theferromagnetic gate 103 so as to contact a source/drain diffusion 406.Because the section illustrated in FIG. 3 is taken at the edge of theferromagnetic gate 103, the cooperating ferromagnetic member is nolonger visible. Within the oxide layer 315 is visible the source line112. Also visible is a portion of the ferromagnetic gate to which a gateline 410 may be joined. In some designs a third drain can be employed.The center drain may be designed to draw more current than the twooutside drains by employing current sources or geometrical layout. Thisincreases the sensitivity of the two outside drains and reduces therequired dynamic range of the input of the sense amplifier.

The cooperating ferromagnetic member 109 of the structure 100 of FIG. 1may be formed by any of several alternative methods.

In one method (describing fabrication of a single device only) a recessis formed in the semiconductor to accommodate the bottom member of theC-clamp, which is then deposited. The surface is planarized, and anoxide layer is grown on the surface of the wafer. The source and drainregions are then patterned and opened up through the oxide to thesilicon. Epitaxial silicon is then grown so as to overlie the bottommember of the silicon using a Silicon-On-Insulator (SOI)-like techniquedescribed, for example, in J. A. Friedrich et al., Jl. Applied Physics,65(4), (Feb. 15, 1989), 1713. Once the bottom member of the C-clamp hasbeen embedded in silicon, the remaining fabrication steps may beaccomplished in a straight-forward manner using conventional techniques.Finally, the ferromagnetic gate is formed in contact with thecooperating ferromagnetic member, also using conventional techniques.

The process flow followed in accordance with the foregoing method isillustrated in greater detail in FIGS. 22 and 23.

In the embodiment of FIG. 1, the cooperating ferromagnetic member isformed early in the sequence of process steps used to form thestructure. Various subsequent processing steps may be required to beperformed at high temperatures. Depending on the specific processingsteps and specific temperatures involved, the ferromagnetic propertiesof the cooperating ferromagnetic member may be affected in a manner thatcannot be readily controlled. Hence, although the C-clamp member of FIG.1 exhibits very desirable magnetic properties, it complicatesprocessing. Various other cooperating ferromagnetic structures may alsobe used. The various cooperating ferromagnetic structures, including thecooperating ferromagnetic structure of FIG. 1, offer different tradeoffswith respect to magnetic properties and process requirements.

Referring to FIG. 5, there is shown a solenoid 500 that may be used as acooperating ferromagnetic structure in place of the cooperatingferromagnetic structure of FIG. 1. Correlate views of the solenoid areshown in FIG. 6, in which a core 501 of the solenoid is shown in dashedlines, and FIG. 7, in which only a coil 503 of the solenoid is shown.The parallelpiped-shaped core of the solenoid is formed of ferromagneticmaterial, and should be electrically conducting (e.g, PERMALLOY). Thecoil of the solenoid is formed of an electrically-conducting material,and should also be ferromagnetic as well. For example, both the core andthe coil may be formed of PERMALLOY. An insulating layer is formedbetween the core and the coil using conventional techniques.

Both the core and the coil are formed layer-by-layer using conventionalsemiconductor processing techniques. In the example of FIG. 5, thesolenoid is formed using a layer structure M₁ I₁ V₁ M₂ I₂ V₂ M₃ I₃ V₃,where M represents a metal (magnetic) layer I represents an insulatinglayer and V represents a via layer, also of (magnetic) metal. Typicallybut not necessarily the via layers are considerably thicker than themetal layers.

For example, in the first metal layer M1, the metal layer may bepatterned to define a stratum of the core and a surrounding trace. Viasmay then be formed in V1, one of relatively large dimensions forming afurther stratum of the core, and another of relatively small dimensionsconnecting to the M1 trace. Note that vias are formed within aninsulating layer. Insulating material therefore fills an interstitialgap between the core stratum and the metal trace. Similar steps are thenperformed for M2 and V2, as will be familiar to those of ordinary skillin the art.

The M3 layer may be processed in like manner as layers M1 and M2. In thecase of the V3 layer, however, only a single via is formed as part ofthe solenoid. The latter via is the final stratum, or "capstone," of thecore.

The process flow followed in accordance with the foregoing method isillustrated in greater detail in FIGS. 24-26. Note that a common maskingoperation is used to define both a trace of the coil and a stratum ofthe core.

Considering the magnetics of the solenoid of FIG. 5, because the coilmakes about 21/2 turns about the core, the current required to switchmagnetic states is less than that required in designs having only asingle turn or, more difficult still, only a partial turn. The solenoidof FIG. 5, however, requires higher switching current than may beavailable in some designs.

Referring to FIG. 8, an exploded view of an alternative solenoid designis shown. FIG. 9 shows an isometric view of the solenoid. FIGS. 10 and11A show rotated exploded and isometric views, respectively. As comparedto the solenoid of FIG. 5, the solenoid of FIG. 8 has fewer turns--11/4instead of 21/2. However, the solenoid of FIG. 8 results in very littlestray magnetic flux. The solenoid of FIG. 8 is further advantageous inthat it allows multiple solenoids to be tightly packed together ininterdigitated fashion. High density is, of course, a prime desideratumin any memory technology.

As with the solenoid of FIG. 5, the solenoid of FIG. 8 is formed inlayers using conventional semiconductor processes. The solenoid of FIG.8, however, exhibits a "metal-on-metal" layer structure--no vias areused. A substantially closed magnetic path is provided by magneticallycoupling the uppermost trace of the coil to the core, either through athin insulating layer if it is desirable to electrically insulate thecoil from the post, or with a continuous connection as shown. FIG. 11Bshows a plan view of the solenoid of FIG. 8 superimposed on a plan viewof a magFET, indicated in dashed lines. The core of the solenoid iscentered over the gate of the magFET. The outer edges of the coil of thesolenoid may be approximately aligned with the center of the source anddrain regions, respectively. In one magnetic state, for example,magnetic flux thus travels down through the core, the ferromagnetic gateand the underlying device channel, out around the gate, up through thecoil, and back down through the core. The magnetizing current can bedirected through just the coil or alternatively through the coil andpost thereby using the post to assist the element switching. If the coiland post dimension are one micrometer or less the magnetic domains mayact as a single domain which can be desirable in many designs.

In the example of FIG. 8, the solenoid is formed using six metal layers.In each of metal layers M1 through M5, a stratum of the core is formed,together with a 1/4-plus-turn trace (the "plus" portion of each traceoverlapping with a similar portion of one or more other traces). At eachsucceeding level, the 1/4-turn trace is rotated (e.g., clockwise) 1/4turn with respect to the preceding trace. The five 1/4-turn traces ofmetal layers M1 through M5 together therefore form 11/4 turns. Ifelectrical isolation of the coil and post is desired, in the metal layerM6, there can be formed a "keeper" member which sits atop the core,separated from it by a thin insulating layer. The M5 trace provideselectrical connectivity, allowing for connection to a common return pathshared by adjoining interleaved solenoids, for example. The M1 traceprovides separate electrical connectivity for each individual coil.

The various cooperating ferromagnetic structures described can becoupled to the ferromagnetic gate of the magFET of FIG. 1 or to aconventional magFET with two or more drains and one or more gates,including possibly a ferromagnetic gate. The various cooperatingferromagnetic structures described normally exhibit two different stablemagnetic states. However it is possible to design the structures so thatthey can be operated on one or more major or minor hysteresis axes toprovide multiple bits of storage per structure. Because of theefficiency of the structures, the switching current required to switchbetween the stable magnetic states is sufficiently small as to besourced by a single, small-dimension MOS device. Furthermore, thevarious cooperating ferromagnetic structures described each cause amagnetic field to be established that is substantially orthogonal to thesense current that flows in the magFET, thereby maximizing carrierdeflection and sensitivity of the magFET. These properties make thepermanent magnetic structures ideally suited for use in magneticintegrated circuit memories as well as other fields of use.

Various elements of the previously described magnetic integrated circuitstructures may be employed separate and apart from acurrent-deflection-type magnetic field sensor such as a magFET. Inparticular, the foregoing structures incorporate what may be regarded asvarious vertical magnetic field generators, where "vertical" isspecified relative to a "horizontal" semiconductor substrate. Each ofthe foregoing magnetic integrated circuit structures will be consideredin turn, particularly with respect to how the structure generates avertical magnetic field, or a vector field which has a verticalcomponent.

C-Clamp

As previously described, the magnetic memories of the 1950's and 60'sused doughnut shaped (toroid) elements of ferromagnetic material forsome or all coil cores. Wires were strung through the center of thecore. Current was run in either one of the two possible directionsthrough the wire so as to magnetize the core in a clockwise orcounter-clockwise direction. The magnetic field in cores is confinedwithin the core itself and there is little magnetic field available toaffect other functions.

It is common in magnetic elements to introduce a gap into the structure.If a gap is small compared to the length of the ferromagnetic material,then the magnetic flux will continue through the gap and allow circuitryto be placed in the gap and hence be in the presence of the magneticfield. The gap can be air or other non-magnetic material, or aparamagnetic material such as silicon.

The C-Clamp magnetic integrated circuit structure of FIG. 1 isconstructed of a ferromagnetic material. The MOS transistor is actuallybuilt into the magnetic gap of the C-Clamp. The gap is small because theC-Clamp also functions as the gate of the MOS transistor and iscontinued underneath the transistor structure.

A MOS transistor needs a metal to provide a work function in order toinvert its channel. This work function can be provided by many differenttypes of metals, and aluminum and polysilicon are usually used. There isno reason, however, why ferromagnetic metal such as nickel-iron cannotbe used.

Using the C-Clamp structure as a gate permits the magnetic field to bedirectly orthogonal to the channel of the MOS device. A magnetic fielddown through the channel will cause some deflection to one of the twodrains, and if the C-Clamp is magnetized in the opposite direction,magnetic flux up through the channel will cause some deflection to theopposite drain.

In order to initialize the C-Clamp to have a clockwise orcounter-clockwise current flow through its structure and gap, the twodrains on the structure are used in conjunction with the C-Clampelectrically functioning as a MOS gate. In other words, thedrain-gate-drain transistor is used as a wire through the core and thebi-directionality of the MOS device permits current to flow in one oftwo directions depending on which drain is at the higher potential. Inorder to write a current in the clockwise direction through element 110of FIG. 1, for example, element 106 could be raised to 7 volts, element105 could be grounded. In order to write the opposite direction, element106 could be grounded and 105 could be tied to 7V. In both cases, itwould be necessary to activate the gate 110 so that a channel is formedbetween source and drain.

Solenoid

The solenoid is a well-known element that produces a relatively constantmagnetic field in its center. It is normally comprised of a group oftightly wound wires on a suitable spool much like sewing thread on abobbin. In order to enhance the magnetic field, a ferromagnetic metalmay be inserted into the center of the spool of wires or act as thebobbin.

It is also possible to use material which is electrically an insulatorbut still ferromagnetic (such as chromium dioxide) to provide selfinsulation of the coil and the port.

The magnetic field is concentrated by the core ferromagnetic element.The magnetic field is directed along the length of the rectangularelement. Magnetic flux lines leave the top of the rectangular core andenter the bottom, or vice versa, depending on the polarity of themagnetic field.

FIGS. 5, 6, and 7, previously described, show a integrated circuitimplementation of a solenoid. FIG. 7 shows the coil of wire where eachturn is formed on a different layer in the vertical direction, and FIGS.5 and 6 show the ferromagnetic core. It is implemented by makingmultiple horizontal partial loops, each on a different layer of theintegrated circuit's metal wiring and having each vertical connectionbetween loops implemented by a via hole which is filled with aconductor. Current flow from bottom to top will produce one direction ofmagnetization; current flow into the top input of the wire and out thebottom of the wire will produce the opposite direction.

The closed magnetic field lines leave the bottom of the ferromagneticmetal and, since the element is placed directly over the gate of the MOSdevice, the magnetic field is essentially orthogonal to its gate.

DNA or Interdigitated Solenoid

The so-called DNA structure shown in FIGS. 8 through 11B is a variationof the solenoid described above. The purpose of the DNA structure is toallow the permanent magnet structure to be built in reduced integratedcircuit area. This is accomplished by having the coiled turns of onepermanent magnet interleave and fit in between the coiled turns of itsneighbor, thereby permitting multiple permanent magnets to share somecommon integrated circuit space, but maintain their magnetic and, ifdesired, also their electrical independence. Like the solenoid, the coilwraps around the ferromagnetic core. Unlike the solenoid, it isconstructed of a group of letter "L" shaped pieces which fit one on topof the other in different integrated circuit layers. The structure shownin FIG. 10 functions like a 11/4 turn solenoid, whereas the solenoid ofFIG. 7 comprises three turns.

Since the magnetomotive force which magnetizes a magnet is proportionalto the number of turns, the DNA has reduced turns but reduced integratedcircuit area requirements. The DNA, like the solenoid, produces avertical magnetic field through its core or post element, which isdependent upon the direction of the magnetizing current through thecoil. The ferromagnetics can be conductive or insulative depending onthe choice of materials. Unlike, the solenoid, the DNA has usefulvariations.

Case 1. In the simplest case the post is isolated electrically andmagnetically from the coil. It then functions like the solenoid earlierdescribed with reduced area requirement and 11/4 turns versus three.

Case 2. In this instance, the coil is electrically insulated from thepost by having the top piece, for example, be placed on a small thinlayer of an insulator such as silicon dioxide, and then resting it onthe post. This structure would be designed so that the insulation layerfunctions as a gap in the magnetic structure. This gap would be designedso that it is adequate to provide electrical insulation but thin enoughto provide magnetic continuity. This feature is desirable when thedecode scheme of circuitry such as a memory requires that the wordslines be isolated from the permanent magnet storage function.

Case 3. It is also possible to have the post and the coil be acontinuous piece of magnetic material. If the ferromagnetic metal isconductive, then electrical conductivity can be provided through thepost and coil so that the coil and post function as a single electricalmagnetic element. One can visualize, for example, the post and coilbeing stretched into one continuous element so as to form a rectangularbar magnet. In suitable geometries, for example sub-micron magnetics, itis possible that the structure will function as a single magneticdomain. The connections to the structure may be made in one of two ways.The coil can be contacted via a contact at the bottom of the coil andtop of the coil or alternatively, two contacts can be made at the bottomof the structure, one to the coil and one to the post. In the lattercase, putting current through a conductive ferromagnetic post willsignificantly reduce the amount of current required to switch thepermanent magnetic structure. This is because the magnetizing force isproportionate to the distance from the magnetizing source. The postmagnetizing current actually acts on the post itself, and is at asmaller distance than the magnetizing force of the coil acting on thepost. The DNA permanent magnet is designed such that winding of the coilreinforces the magnetizing force of the post on the post itself in orderto assist the switching action.

In a preferred embodiment, the foregoing cooperating ferromagneticstructures are made from PERMALLOY which is a nickel-iron mixture. Thecoercivity of the material is a measure of the stability of thestructure and an indication of the amount of magnetizing currentrequired to switch the structure. The structures are normally designedfor 15 to 50 oersteds and a magnetic field across the channel of 50 to300 gauss. The coercivity and the magnetic field are then significantlyabove the earth's magnetic field of about 0.5 gauss and 0.5 oersted andsufficiently stable to effect reliable data storage. The coercivity ofnickel-iron can be adjusted by numerous methods including adding othercompounds such as cobalt and by numerous manufacturing techniquesincluding adjustment of the substrate temperature during filmdeposition. The magnetomotive force required to switch the structure isprimarily a function of the gap length. A very small gap is desirable toallow small values of current to switch (write) the structure.

Single-Domain Post or Tube

This magnetic integrated circuit structure may be imagined as amicroscopic vertically oriented bar magnet, sufficiently small to behaveas a single magnetic domain. The coercivity of such a structure may bemade adjustable by forming a non-ferrous cylinder and depositing aroundthe cylinder a ferrous skin of a desired thickness. The relative massesof the non-ferrous core and the ferrous skin influence coercivity. Theresulting tube has an elongated geometry. Because of the single domainnature of the tube and its elongated geometry, it can exist in only oneof two stable magnetic states and requires that a destabilizing currentbe applied to switch between the two states. The field intensityobtained using such a magnetic tube may be much greater than thatobtained using most other magnetic structures. This greater fieldintensity translates into a larger signal that may be more readilysensed. A greater field intensity also means faster operation.

Integrated circuit memories using the foregoing magnetic integratedcircuit structures will now be described.

The foregoing magnetic integrated circuit structures may be used toadvantage within existing magnetic semiconductor memory designs andmagnetic field sensor designs, for example those of U.S. Pat. No.5,068,826 (the Matthews patent) and U.S. Pat. No. 3,714,523 (the Batepatent), both of which are incorporated herein by reference. As appliedto magnetic semiconductor memories, these designs, as originallyconceived, suffer from the difficulties previously described (largewrite current, large cell size, stray magnetic coupling). The efficiencyof a magnetic structure is directly related to the ratio of the lengthof flux paths in the core as compared to the length of the gap. Theratio should be as large as possible. In the foregoing magneticintegrated circuit structures, this ratio is large, an important factorin enabling the disadvantages of the prior art to be overcome.

Referring to FIG. 13A illustrating a prior-art memory cell in accordancewith the Matthews patent, a Sensor portion 10 of the memory cellcomprises a semiconductor Hall bar fabricated in a silicon substrate. Adrive current, I_(d), enters the Hall bar at p+ drain region 14 andexits the bar at p source region 15. The spaced-apart regions 14 and 15define a channel 17 therebetween. The combination of regions 14, 15channel 17 and an overlying polysilicon gate (not shown in FIG. 1) forman ordinary p-channel metal-oxide-semiconductor (MOS) device. Theapplication of a low potential (e.g., 0 Volts) to the control electrodeof a p-channel MOS device causes an inversion layer to be generateddirectly below the substrate surface in channel 17. This inversion layerprovides a conductive path for the drive current I_(d) to flow. In theparticular implementation of FIG. 1, external voltage source 11generates drive current I_(d).

A pair of bipolar transistors are integrated into the Hall sensor asbuffer amplifiers. These transistors are shown in FIG. 13A astransistors 28 and 29. The base-emitter regions of NPN transistor 28 areindicted by regions 19 and 21, respectively, while the base-emitterregions for the NPN transistor 29 (located on the opposite side of thechannel 17) are indicated by regions 18 and 22, respectively. The n-typesubstrate acts as a common collector for both transistors. As isdescribed in the Matthews patent, the Hall voltage generated in theinversion layer of the MOS transistor becomes ohmically coupled to baseregions 18 and 19. In this way, the voltage V_(Hall) produces adifference potential across emitters 21 and 22, which is then detectedby voltage sensor 12.

Referring to FIG. 13B, there is shown in cross-section a magneticsemiconductor memory cell like that shown in FIG. 3 of the Matthewspatent but incorporating a solenoid 33, shown in outline, of one of thetypes previously described in relation to FIGS. 5 and 8. The illustratedcross-section corresponds to Section A-A' in FIG. 13A. Spaced-apart p+regions 14 and 15 are shown fabricated in an n-type substrate 27.Regions 14 and 15 are fabricated using either ordinary diffusion or ionimplantation techniques. A polysilicon gate 24 is formed directly abovethe channel 17. Gate 24 is insulated from substrate 27 by the gate oxide26 below, and from magnetic patch 33 by a dielectric insulation layer 25above. Ohmic contact to regions 14 and 15 is provided by members 37 and38, respectively. In a BiMOS process, members 37 and 38 comprise dopedpolysilicon; however, ordinary metal, or other conductive materials, mayalso be used.

During normal read operations of the memory cell of FIG. 13B polysilicongate number 24 is grounded to create a p+ inversion layer 35 in channel17 of the sensor. Once inversion layer 35 has been formed, drive currentmay flow between source/drain regions 14 and 15. A voltage, V_(Hall), isproduced across layer 35 (directed either into or out of the page) inresponse to magnetic field 23.

In FIG. 13C a cross-sectional view of the magnetic memory cell of FIG.13A is shown along cut lines B-B'. FIG. 13C illustrates how the bipolarjunction transistors are integrated with the MOS Hall sensor. Basically,this is achieved by forming base regions 19 and 18 directly adjacent tothe channel region 17 so that inversion layer 35 extends all the way towhat normally is the base-collector junction of transistors 28 and 29.

In FIG. 13C, p-type region 19, n+ region 21 and n-type substrate 27 formNPN transistor 28. NPN transistor 29 is formed by substrate 27, p-typebase region 18 and n+ emitter region 22. As discussed above, whencurrent I_(d) flows across inversion layer 35, a Hall voltage isdeveloped across the transverse portion of channel 17. The p-typeinversion layer 35 extends completely across channel 17 where it makesohmic connection to each of the p-type bases 18 and 19. Thus, thevoltage V_(Hall) developed in inversion layer 35 is ohmically coupled tobase regions 18 and 19 of transistors 29 and 28, respectively.

The solenoid 33 significantly increases the magnetic flux path length ofthe Matthews prior-art memory cell, thereby significantly increasing themagnetic efficiency of the cell and reducing the amount of currentrequired to write information into the cell. The size of the cell may becorrespondingly reduced. At the same time, the solenoid reduces straymagnetic flux and its influence on adjacent cells.

Referring to FIG. 14, there is shown a magnetic semiconductor memorycell using a magnetic field sensor like that shown in the Bate patentbut incorporating a solenoid 33, shown in outline, of the typepreviously described in relation to FIG. 8. The memory cell of FIG. 14offers the advantage that a small current imbalance may be sensed and aresulting voltage amplified to cause an output signal to undergo a rapidand pronounced voltage swing. More particularly, in the embodimentillustrated in FIG. 14, gates G₁ and G₂ and drains D₁ and D₂ are at thesame potential, determined by the voltage source shown schematically at34, when no magnetic field is present. It may be seen that thisstructure in essence defines two separate FET's, one device includingD₁, G₁ and the source S₁ and the other device including D₂, G₂, and thesource S₁. The gates of these two devices are formed sufficiently closeto each other such that they interact in response to a magnetic field toproduce an enhanced output signal as follows.

When a magnetic field is applied so that it is directed out of the sheetof the drawing, as schematically illustrated by the circled arrow tipsat 36, holes in the inversion layer under G₁ are diverted from left toright to the inversion layer under G₂ by the force due to the combinedeffects of the electric field and the magnetic field. As a result, thedrain current I_(D2) increases while the current I_(D1) decreases. Thecross-connections of the drains D₁ and D₂ to gates G₂ and G₁,respectively, provides positive feedback which produces an enhancedoutput signal. Varying the external load resistance R_(L) affects thesensitivity and stability of the FET magnetic field detector. Ascompared to other memory cells, the memory cell of FIG. 14 formed bycombining a cooperating ferromagnetic member with the prior-art magneticfield sensor of Bate requires two separate control lines to control thepotentials of the two separate gate portions. However, the sensorportion of the cell exhibits high sensitivity. The cooperatingferromagnetic member would be placed over the two gates and not beelectrically or magnetically attached to the two gates.

Other novel and advantageous magnetic semiconductor memories using theforegoing magnetic integrated circuit structures will now be described.

Referring to FIG. 15, there is shown a two-word by two-bit (four-cell)storage cell array constructed using the magnetic integrated circuitstructure of FIG. 1. Each storage cell has the same construction asdescribed previously in relation to FIG. 1, i.e., each storage cellincorporates a magFET having a ferromagnetic gate and a cooperatingferromagnetic member that is magnetically coupled to the ferromagneticgate to form a C-shaped magnet. Each cell requires connections to a wordline, two data lines, and a ground line, or reference potential line.The ground line may be shared by all cells adjacent to it. Layoutefficiency is therefore achieved by having different halves of the 2×2array mirror one another about a line of symmetry. The sources of allthe cells are then connected in common to a single ground line. Twoopposing cells directly opposite one another across the line of symmetrymay share a single source, furthering compactness. In the illustratedembodiment, one source S1 is shared by cells 1500 and 1510, and anothersource S1 is shared by cells 1501 and 1511. Alternatively, the cells mayeach have separate sources, which may be desirable in certain types ofdecoder schemes.

Data lines run "across" memory words, and select lines run "along"memory words. The select line connects in common to the gates ofdifferent cells within a single memory word. Hence, the gates 00G and01G of cells 1500 and 1501 are joined by a R/WO select line SEL0. Thegates 10G and 11G of cells 1510 and 1511 are joined by a R/W1 selectline SEL1. In the opposite direction, one line D0 of a differential dataline pair (DATA 0) connects corresponding ones (e.g., drains 02D1 and11D1) of the two drains of respective cells 1500 and 1510, and the otherline D0b of the same differential data line pair connects correspondingother ones of the drains of the same cells (e.g., drains 01D2 and 11D2).Similarly, one line D1 of the differential data line pair (DATA 1)connects drains 00D1 and 10D1, and the other line D1b of the same pairconnects drains 00D2 and 10D2.

Assume that each individual gate is magnetically polarized so as toproduce in a corresponding respective channel region a vertical magneticfield in either the upward direction or the downward direction. In orderto read information stored in Word 0, the select line S0 is driven to alevel of, say, 2V, sufficient to cause the respective magFETs of cells1500 and 1501 to conduct. The differential line pairs are coupled torespective sense amplifiers. Through the current deflection mechanismpreviously described, the respective magFETs will sense the polaritiesof the magnetic fields stored within the respective magnets of cells1500 and 1501, causing these polarities to be reflected in the voltagepolarities carried on the respective differential line pairs.

In some designs, it may be desirable to hold the voltage in the drainsduring the READ operation at approximately same voltages to maximize thesignal and avoid parasitic transistor action among the drains or to keepthe drains at a relatively high voltage to cause significant electricfield across the source-drain region. These objectives can beaccomplished, as shown in FIG. 27, by simply adding a buffer circuit ina cascade configuration that translates the current difference in thedrains into a subsequent voltage. For example the emitter of bipolartransistor or the source of other MOS device could be tied to the drainswhile having the base or gate held at constant potential. A currentdifference in the magFET drains will now show in collector or drainvariations while substantially maintaining selectively high or stablevoltages in the magFET.

In order to write a data bit into a cell, a magnetizing current must bepassed "through the mouth of the C", in one direction to store a logic 0and the opposite direction to store a logic 1. To write a data bit, thetwo drains of a cell function as a transistor to pass current in one oftwo possible directions from one drain to the other. The direction ofthe lines of flux and the consequent polarity of the magnetic fieldproduced reverse according to the direction and carrier polarity of themagnetizing current. Given a C-shaped magnet having the same orientationas the letter C on a page, a current in the direction into the pageproduces flux lines in a clockwise counter-clockwise direction dependingon whether the carriers are holes or electrons. The upper "jaw" orpolepiece of the C-shaped magnet is therefore magnetized as a North poleand the lower polepiece is magnetized as a South pole. A current in thedirection out of the page produces the opposite magnetization.

In some designs, the required magnetizing current may be substantial, 3mA for example. A small-size MOS device of suitable width to lengthratio is able to provide such a current if the gate voltage of thedevice is raised to a sufficiently high voltage, e.g., 7V. Therefore, towrite information into a memory word, the select line for that word israised to what would typically be a higher-than-normal voltage level.Differential voltages of the appropriate polarities are placed on thedifferential line pairs, causing the magnets of the respective cells tobe magnetized with corresponding polarities to therefore represent logic0s and logic 1s as desired.

The 2×2 storage cell array of FIG. 15 is shown in plan view in FIG. 16.Cross-section views indicated in FIG. 16 are shown in FIG. 17 and FIG.18, respectively. FIGS. 16, FIG. 17 and FIG. 18 correspond in largemeasure to FIG. 2, FIG. 3 and FIG. 4 previously described and willtherefore not be further described.

In the case of the memory cells of FIG. 15, the cooperatingferromagnetic member of each cell underlies the ferromagnetic gate andtogether with the ferromagnetic gate forms a C-shaped magnet. In placeof such a cooperating ferromagnetic member, other cooperatingferromagnetic members may be used as previously described. Referring toFIG. 19, there is shown a two-word by two-bit (four-cell) storage cellarray using as the cooperating ferromagnetic member the solenoid of FIG.5. Within each cell, a solenoid like that of FIG. 5 is formed on top ofand magnetically coupled to the gate of the magFET.

As compared to the storage cell array of FIG. 15, in the storage cellarray of FIG. 19, three data lines are provided for each cell. Thedifferential lines, D0 and D0b for example, function in like manner asin the storage cell array of FIG. 15. The third data line, DOW, forexample, is pulled high or low during a write operation depending on thelogical value to be written, thereby determining the direction ofcurrent through the solenoid. The source is not grounded as in theprevious embodiment, nor does write current flow from drain to drain asin the previous embodiment. Instead, write current flows from drain tosource through a coil of the solenoid.

As in the case of the storage cell array of FIG. 15, in the storage cellarray of FIG. 19, two opposing cells directly opposite one anotheracross the line of symmetry may share a single source as shown in FIG.19, furthering compactness. Alternatively, the cells may each haveseparate sources.

An equivalent circuit of the memory cell of FIG. 19 is shown in FIG. 20.The source of a magFET 2001 is coupled to the coil of a solenoid 2003 ofone of the types shown in FIG. 5 and FIG. 8. The other end of the coilis coupled to a data write line DOW. The data write line may be groundedor raised to a logic high (e.g., 5 v) through a switch 2005. The drainsD₁ and D₂ of the magFET are coupled to differential data lines D0 andD0b, respectively. The differential data lines may be grounded, raisedto a logic high, or floated by means of ganged switches 2007A and 2007B,respectively. The differential data lines are also coupled to adifferential amplifier (not shown). The gate of the magFET may begrounded to deselect a decoded row, raised to a relatively low voltagelevel (e.g., 2 v) to read, or raised to a high voltage level (e.g., 7 v)to write.

In operation, in order to write a data bit to the memory cell,magnetizing current is caused to flow in one of two possible directionsthrough the coil of the solenoid and through the magFET. For example, towrite a logic high value to the memory cell, the switch 2009 is switchedto the uppermost position so as to apply 7V to the gate of the assumedn-channel magFET shown. At the same time, the switches 2007A and 2007Bare switched to the uppermost position to apply 5V to the drains D₁ andD₂, and the switch 2005 is grounded. A current I_("1") is thereforecaused to flow right to left through the coil or the coil and port ofthe solenoid. To write a logic low value, the switches 2005, on the onehand, and 2007A and 2007B, on the other hand, are placed in oppositepositions. That is, the switches 2007A and 2007B are grounded, and theswitch 2005 is raised to 5V. A current I_("0") is therefore caused toflow right to left through the coil of the solenoid.

It is also possible to avoid the "read disturb" of having some readcurrent go through the coil or alternatively have the gate voltage forthe read and write be the same by adding an additional selection wirewhich contacts the source "S".

The read operation is typically performed by putting switch 2009 in the2V position and by putting switches 2007A and 2007B in the FLOATposition and switch 2005 in the grounded position. MagFET 2001 is as aresult biased ON and approximately equal drain current will flow throughfrom +V into R+ and R- if there is no magnetic field. A magnetic fieldorthogonal to the gate of 2001 will cause either more or less current toflow through R+ and R- depending on the direction of the magnetic field.The current difference is then sent to the differential amplifier forsensing. The amount of current sent through element 2003 during read,the "read disturb current" is significantly below what is required toswitch it if the read operation is designed to be a nondestructive readout. The "read disturb" is typically held to less than 30% of the writecurrent to provide a safety margin by having a lower gate voltage duringread than during write.

Prior art magnetic memory elements pose considerable difficulty inswitching between states and in reliably sensing the difference betweenstates. Switching between states has required considerable current. As aresult, cycle time is prolonged. Distinguishing between states hasrequired very sensitive circuitry because of the very small signalstrength produced by a typical magnetic memory element.

The foregoing difficulties may be overcome by using as the magneticmemory element a magnetic post or tube of sufficiently small dimensionsas to constitute a single magnetic domain.

Today's semiconductor lithography is at 0.25 micron minimum dimensionand heading rapidly for 0.1 micron. Magnetic structures produced usingstate-of-the-art lithography can now, for the first time, be singledomain, enabling enhanced sense signals and increased competitiveness,to the magnetic semiconductor memory. Other technologies such asferroelectric and DRAM capacitors do not scale well to smallerdimensions, and they may face a physical limit, since their basic unitcell at the atomic level requires more atoms. Magnetic devices appear toget better as they get smaller, providing a viable path to deepsub-micron size memories.

Magnetic permanent structures are defined herein as those having a sizeroughly 1 micron or larger. The remanent magnetic field of macromagneticstructures, after the magnetizing current is removed, is determined bythe so-called magnetic B-H characteristics, and by the ratio of thelength of the magnetic structure to the length of the non-magneticportion or gap. The smaller the gap the stronger the remaining field.The relationship is linear. A so-called load-line is usually drawn onthe second quadrant of the B-H curve to predict the static permanentmagnet's field once the magnetizing current is removed.

For example, in a horseshoe shaped magnet, the B-H loop might produce aB of 10,000 gauss when the magnetizing current H is maximum. Theremanent field with no magnetizing current and no gap might be 7500gauss. If the gap is one tenth the length of the structure, the fieldwould be about 750 gauss in the gap.

In a micromagnetic structure having a size less than a value roughlybetween 0.5 and 1.0 microns, the macromagnetic rule needs to bemodified. The statistical switching or partial switching of a group ofdomains is what causes the curves in the B-H plots. As size gets below 1micron and the size of the structure starts to approach the size of adomain, the structure starts to "have room" for just a single domain andnot have room for multiple domains. The B-H plot consists of onlysingular points, and load-line analysis doesn't apply.

The magnetic field, as in the macromagnetic case, falls off as the gapgets longer and is dictated by magnetostatics, but the remanence valueappears to remain at 7500 gauss, not the 750 gauss above.

This behavior makes intuitive sense, in view of tremendous bindingforces at the atomic level holding atoms together as evidenced by atomicbombs releasing this energy. As structures get smaller, the first signsof these increases may be observed, marking the boundary ofmicromagnetics and the single domain region.

A greatly enlarged view of a single domain magnetic memory element isshown in FIG. 27. The magnetic memory element is formed on a substratehaving a magFET formed therein, the magFET including (as described inrelation to previous embodiments) a source (mostly obscured in the viewof FIG. 27), dual drains, a gate oxide layer and a (non-ferrous) gateelectrode. Drain wires are joined to respective drain regions bycontacts. A source contact is shaped so as to form above the gate aplatform on which is formed a non-ferrous post having a ferromagnetic(e.g., NiFe) skin. To the top of the post is joined a source wire. Apair of orienting wires run alongside the post on either side,preferably about mid-way up the post. Current is driven through theorienting wires, always in opposite directions, such that the orientingwires form a pseudo turn with both ends of the wires meeting,hypothetically, at a distance of infinity. (In some embodiments, theorienting wires may actually form a physical loop.) The manner in whichinformation is written to and read from such a magnetic memory elementis described hereafter.

A 2×2 array of magnetic post memory elements is shown in FIG. 28. In alarge array of such memory elements, sharing of structures betweenneighboring elements is possible as described in relation to thepreceding embodiments.

FIG. 29 is a list of process flow steps for forming a magnetic memoryelement like that of FIG. 27. The illustrated process is exemplary onlyand sets forth numerous details not necessary to practice of theinvention.

In steps 1-6, the cantilevered pedestal on which the post sits isformed. The vertical portion of the formed first, followed by thehorizontal portion. In steps 6-9, the non-ferrous post is formed byplating. Because of lithography limitations, the post is initiallyoversized. In step 10, the post is etched down to a smaller size arerequired for single-domain magnetic behavior. The post is then platedwith NiFe or other suitable magnetic material (step 11). During step 13,a thick insulating layer surrounding the post is formed of a suitableinsulating material, e.g., SOG, polyimide, etc. The insulator is thenpatterned to make contact to the top of the post. The NiFe is removedfrom the top of the post for this purpose. The remaining steps in thefabrication sequence are largely conventional.

The coercivity of the magnetic tube structure may be adjusted byadjusting the thickness of the magnetic skin. As a result, the magneticmemory element is readily scalable to smaller geometries as lithographictechniques improve. Were the post formed entirely of magnetic material,the coercivity of the resulting structure would likely be too great toallow it to be switched.

The combination of very small, single-domain size and a relatively largeaspect ratio results in uniquely desirable properties. Current levelswithin any reasonable expectation operate to switch the state of themagnetic tube only when the magnetic tube is destabilized by runningcurrent through it. With current flowing through the magnetic tube, itsstate may be readily changed by running modest current levels throughthe orienting wires. When the magnetic tube is switched, the singledomain nature of the magnetic tube produces a signal that is typically5-15 times stronger than signals produced by conventional magneticmemory elements.

To write information to a magnetic memory element like that of FIG. 27,the gate line is activated and a destabilizing flow of current is drivendown the post and through the underlying magFET, from the source to thedrains. As a result, the magnetic post enters a meta-stable magneticstate which, when the destabilizing flow of current is removed, willrevert to one of two stable magnetic states. In these two stablemagnetic states, the magnetic post functions as a vertical magneticfield generator, in one of the states generating a magnetic fluxdownward through the gate of the magFET and in the other of the statesgenerating a magnetic flux upward through the gate of the magFET. Areal-world analogy is helpful in understanding operation of the magneticpost. Hold a pencil vertically on the table resting on its point. Thisis the magnetic equivalent of destabilizing it. Which way it falls to aparticular stable state (left or right or front or back) is a functionof the wind, initial alignment, etc. To make the pencil fall aparticular way a slight additional force or wind (analogous to theorienting wire current) can be used once the pencil weight (analogous tothe coercivity) has been negated by the vertical position (which isanalogous to the destabilizing current.)

By way of additional comparisons, in another magnetic storagetechnology, the media is heated via lasers to the Curie temperaturewhere it loses magnetism and is then allowed to cool in the presence ofan orienting field. Basically the coercivity is knocked out of thematerial by heating it and then it is oriented as desired. The describeddestabilizing current is analogous to the heating since it puts acircular magnetic field around the post (when viewed from the top), yetpost geometry dictates an up/down or down/up stable state with noexternal destabilization, thus the field must fall from circular tovertical. Absent the orienting wires, the magnetic post would revert toone of the two stable magnetic states in an uncontrolled manner. Thefinal state might, for example, be determined by many different straymagnetic fields and disturbances such as the Earth's magnetic field,state of the neighbors, etc. The orienting wires allow the magnetic postto be influenced to transition to a desired one of the two stablemagnetic states. Before the destabilizing flow of current has ceased andstate transition has begun, an orienting current is driven in onedirection or the other through the pseudo loop; i.e., assuming that theorienting wires run in a horizontal direction, either current is drivento the right in a first orienting wire and to the left in the otherorienting wire, or current is driven to the left in a first orientingwire and to the right in the other orienting wire. In this manner,either a logic 1 or a logic 0 may be written.

Information is read from the magnetic post memory element in the samemanner as described in relation to the preceding embodiments. The readcurrent that flows through the magnetic post is less than a thresholddestabilizing value such that read-out is non-destructive.

Unlike previous embodiments, in the present embodiment, current flowthrough the magFET is required in only one direction. Referring to FIG.30 therefore, an equivalent circuit diagram of the magnetic post memoryelement and associated circuitry differs from the circuit of FIG. 20 inpart in that the drain lines and the source line are switched to only asingle source of potential. (The destabilizing current functions to formcircular fields--it doesn't matter which direction of rotation thecircular fields have. Unlike FIG. 20, in FIG. 30, orienting wires areprovided to set the direction--up/down or down/up--of the magnetic fieldin the steady-state condition.) Furthermore, the arrangement of the"solenoid" is completely changed. Instead of current flowing through thecoil of the solenoid as in FIG. 20, current flow through the core of thesolenoid. Current is separately driven through the coil (pseudo coil, ororienting wires) in either of two directions.

It will be appreciated by those of ordinary skill in the art that thepresent invention may be embodied in other specific forms withoutdeparting from the spirit or essential character thereof. The foregoingdescription is therefore considered in all respects to be illustrativeand not restrictive. The scope of the invention is indicated by theappended claims rather than the foregoing description, and all changesthat come within the meaning and range of equivalents thereof areintended to be embraced therein.

What is claimed is:
 1. A method of forming a magnetic memory element,comprising the steps of:forming a cantilevered pedestal; forming anon-ferrous post on the cantilevered pedestal; and forming a ferrousskin about the non-ferrous post.
 2. The method of claim 1, whereinforming the non-ferrous post comprises plating the non-ferrous post to apredetermined height.
 3. The method of claim 1, wherein forming theferrous skin comprises electrolytically plating the ferrous skin ontothe non-ferrous post.
 4. The method of claim 1, wherein the cantileveredpedestal is formed in predetermined relationship to a magFET having asource, at least one drain, and a gate electrode.
 5. The method of claim4, wherein the cantilevered pedestal so as to contact the source andoverhand the gate electrode of the magFET.